290 
ON CHEMISTRY. 
nature of the elasticity of the confined air, by the sensation excited 
on the finger, h will be thereby evident, that, were it not for the 
resistance of the membrane, the air within it, would extend outwards, 
and occupy more space; and in fact, an increase of heat will, some¬ 
times cause the expansive force to burst the bladder with a consider¬ 
able explosion. By removing the bladder to a distance from the 
fire; the air will contract, the membrane will no longer be pressed 
outwards, and therefore, will soon resume its loose and flaccid ap¬ 
pearance. 
Persons who possess an air-pump, and the apparatus which instru¬ 
ment-makers term —“ a bladder and weights" —can easily satisfy 
themselves that, a very minute portion of air in a sheep's bladder, 
(perhaps not so much as might be contained in the egg of a pullet,) 
is capable of not only distending the bladder to its utmost dimen¬ 
sions, but of raising a set of leads, weighing from twelve to fifteen 
pounds. The apparatus must be placed under the receiver of the 
pump, from which the air is to be withdrawn. The external pressure 
being thus removed, an effect equivalent to the action of heat is pro¬ 
duced, and the air within the bladder expands, forcing up the mass 
of incumbent weights. This is a beautiful, convincing, and most 
important experiment; but like all the others, leaves the mind in¬ 
volved in wonder at the mysterious operation of the exciting cause. 
Philosophers then, arc correct, when they assert that “ the atmos¬ 
phere is an elastic fluid which invests the earth;—that it moves and 
presses equally in all directions, and therefore, rushes in, and fills 
every place not previously occupied by a more solid substance.” 
Air possesses weight as well as elasticity; and this can be proved 
by means of an accurate and delicate balance. It has been ascer¬ 
tained that, when the Barometer stands at 30 inches, and the Ther¬ 
mometer at 60 degrees, 100 cubical inches weigh about 31 grains. 
At this calculation, 1000 cubical inches will weigh 310 grains, and 
a cubic foot (or 1728 square inches) will, in round numbers, be es¬ 
timated at 535 grains : these weights refer solely to air at the surface 
of the earth. 
But as its particles press upon one another, so, as the distance 
above the surface increases, this pressure diminishes, and the density 
of the air becomes less. Hence, as we ascend in the aerial region, 
the bulk of air cannot be of the same weight, or exert the same elas¬ 
tic force. It becomes more attenuated,—that is, —the same quantity 
occupies an enlarged space; and in very elevated situations, retains 
scarcely enough of elastic power to expand the lungs; and respira¬ 
tion therefore, becomes laborious. We have no means to determine 
